1. Field of the Invention
This invention relates to laser radar systems (ladars). Specifically, the present invention relates to synthetic aperture ladar systems.
2. Description of the Related Art
Long range imaging or mapping has become of great interest for both defense and commercial applications. In defense applications, high-resolution target imaging allows for target identification at safe ranges beyond weapon capabilities. For defense and commercial applications, there is a need to perform terrain mapping for high-resolution topography.
High resolution imaging at long ranges is becoming a commercial priority as well. High resolution imaging will allow high flying aircraft to determine the content of open bay trucks, or whether people are carrying weapons or tools. Other applications such as NASA's desire to map possible landing sites for unmanned crafts, drive resolution requirements to just a couple of inches at ranges greater than 100 km.
High resolution long range imaging would also be useful for numerous additional applications including Homeland Security and terrain surveillance.
Passive optical (infrared) approaches have been used in the past for this application as well as synthetic aperture radar (SAR). Infrared approaches are limited by the physical aperture size of the optics.
Conventional SAR with its associated long wavelength cannot achieve the desired high resolutions. Also, the time required to span the much larger synthetic aperture needed for high resolution make it impossible to image targets that have any motion.
U.S. patent application Ser. No. 10/020,730 filed Oct. 30, 2001 by M. J. Halmos entitled SYNTHETIC APERTURE LADAR SYSTEM USING INCOHERENT LASER PULSES, the teachings of which are hereby incorporated herein by reference, discusses the use of a uniformed spaced set of pulses from a mode locked laser and also the use of an incoherent single pulse waveform.
Unfortunately, in certain applications, where the target range is very long, a single pulse is not practical due to the large amount of energy required. These energies could cause optical damage. The solution would be to spread the energy into multiple pulses as with the mode lock waveform. However, a drawback of this waveform, is that the repeating pulses may cause an inherent range ambiguity due to the repeating nature of the pulses. A third approach commonly used in synthetic aperture radar is to use the Linear FM chirped waveform. However, this waveform is difficult to obtain due to the high time-bandwidth products required and the lack of high bandwidth optical modulators. Also, one would like to modulate a laser intra-cavity where a linear phase change is augmented with a linear frequency chirp. However, the maximum chirp that one can practically get is limited by the laser mode spacing c/(2L) (c=speed of light and L is the resonator optical length).
Hence, there is a need in the art for an improved system or method for long range, high resolution imaging.